why list can't be initialized with array, without arrays.aslist()?

Question

Arrays.asList() is declared as below.

@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

So, as you know, to initialise a concrete List with an array, you can do this:

String[] strings = new String[] {"hello", "world"};
List<String> list = new ArrayList<>(Arrays.asList(strings));

But what about this?

List<String> list = new ArrayList<>(new String[]{"hello", "world"});

I expected it would works, but didn't.

I understand why. It's because one of the constructors of ArrayList demands Collection class.

Then, what about Arrays.asList() ?

Varargs are compiled as array, so maybe that method would be compiled as below.

public static <T> List<T> asList(T[] a) {
    return new ArrayList<>(a);
}

But it actually returns ArrayList object. Why is this possible?

Sweeper · Accepted Answer · 2020-10-30 01:57:37Z

Indeed, Arrays.asList is implemented as you said, but ArrayList doesn't actually refer to the java.util.ArrayList that we all know and love. It refers to a private inner class in the Arrays class. From the first link, just under the declaration of asList, you will see:

/**
 * @serial include
 */
private static class ArrayList<E> extends AbstractList<E>
    implements RandomAccess, java.io.Serializable
{
    ...

new ArrayList<>(a) is referring to the above class. The above class has a constructor that takes an E[]:

    ArrayList(E[] array) {
        if (array==null)
            throw new NullPointerException();
        a = array;
    }

But the java.util.ArrayList class doesn't have such a constructor, so it is no wonder that you can't do:

List<String> list = new ArrayList<>(new String[]{"hello", "world"});

Another big difference between java.util.ArrayList and java.util.Arrays$ArrayList is that you can add/remove elements to the former, but you can't to the latter. e.g. This throws an exception:

Arrays.asList("a", "b", "c").add("d");

Eugene · Accepted Answer · 2020-10-30 15:23:26Z

2

I absolutely never understood why such a constructor does not exist in java.util.ArrayList to begin with and the decision to name an inner class ArrayList too - is even more interesting. So you are simply looking in the wrong place, nothing more.

They did add such a construct (!= constructor), but only in java-9 and only to immutable collections:

List<String> list = List.of(new String[]{"one"});

edited Oct 30, 2020 at 15:23

answered Oct 30, 2020 at 2:02

Eugene

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2 Comments

daniu Over a year ago

Nitpicking, but that's not a constructor, but a factory method. A pretty pointless one, considering there already is one exactly like it - namely Arrays.asList() .

DuncG Over a year ago

@daniu Note that Arrays.asList() is modifiable - and set/replaceAll affects the content of the original array whereas List.of content is a not modifiable copy of the original array.

Shubham · Accepted Answer · 2020-10-30 01:40:41Z

0

Arrays.asList() is read only collection.

To new ArrayList(...) you have to pass a collection as an argument.

A collection in java is a List, Set

answered Oct 30, 2020 at 1:40

Shubham

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